Various liquid materials, such as, for example, isopropyl alcohol and hydrofluoric acid, or diethylsilane are used in the semiconductor manufacturing process. These liquid materials are filled into a container having a predetermined volumetric capacity and are often transferred by delivering inert gases having predetermined pressure into the filling container. At this moment, in the actual operation state of the manufacturing process, in order to control the liquid material feed rate or in order to secure supply, a method is often applied wherein the liquid material residual volume is detected directly by means of various sensors provided on the filling container side while processes such as liquid material gravimetric measurement are also carried out. Usually, a mass flow controller (MFC) is used in the liquid material gravimetric measurement, and observation sensors such as liquid level indicators are used for detecting the liquid material residual volume. At this moment, in case the liquid material residual volume is detected directly, observing the state of the liquid material inside the container becomes essential, whereas for the observation sensors, adequacy of the residual volume detection and precision high level are demanded, and at the same time, sensors which enable detection in conditions of complete non-contact with the liquid material are preferable in order to prevent liquid material contamination or sensor contamination.
The following means exist as conventional measurement methods for liquid level observation: a means of measuring the liquid level by providing float-type sensors inside the container, a means of using a gravimeter made of load cells and the like, or a means of detecting the liquid level by means of ultrasonic systems measuring the distance from the top of the container to the liquid level detecting the ultrasound wave reflecting on the liquid level.
In concrete terms, as shown in FIG. 8, a float-type liquid level indicator can be given, consisting of guide pipe 111 installed in standing position in the liquid, float 112 floating at liquid level inside guide pipe 111, optical attenuation plate 113 installed in standing position on the surface of float 112, and an optical detector 100 having both light emission and light reception means installed on the sides of guide pipe 111 to the right and left of the light path plane of the optical attenuation plate 113. The transmitted light volume increases and decreases according to the up and down motion of the optical attenuation plate 113 continuously modifying the optical transmittance in the length direction (lengthwise direction) thereof, moving together with the up and down motion of float 112. Consequently, the liquid level can be detected by means of the increase and decrease of said transmitted light volume. (See for example Japanese published unexamined application No. 1987-001124.)
Moreover, as shown in FIG. 9, a liquid level indicator using gravimeter 201 can be given. Container 202 is placed above gravimeter 201, liquid 203 is stored in this container 202, heated by means of heater 205 connected to power supply 204 located in this liquid 203 and structured so that boiling is obtained, gas 206 generated by boiling is ejected by means of gas delivery unit 207 located on the upper part of container 202, and delivered to the required consumption pipe via jabara tube 208. Liquid 203 is supplied via jabara tube 210 connected to liquid supply unit 209 located on the lower part of container 202, in order to supply liquid 203 corresponding to the liquid amount decreasing by means of supplying this gas 206 to the exterior. In supplying this liquid 203, it is necessary to perform supply so that the volume of liquid 203 in container 202 becomes nearly constant; therefore, as a method of observing the liquid volume, a method is used wherein the weight of container 202 filled with liquid is measured, liquid volume is estimated from this weight, and the liquid level is determined from the estimated liquid volume thereof. (See for example Japanese published unexamined application No. 2000-046631.)